(a) Field of the Invention
This invention relates to a photopolymerizable screen printing ink composition which is very useful especially as a permanent protective film for use in printed circuit boards, in particular to a non-volatile ink composition which, even in a thick film state, is curable upon short time exposure to ultra-violet irradiation thereby to form a film which possesses, in abundance, various characteristics such as electric insulation, heat resistance, hardness, flexibility, adhesion and solvent resistance.
(b) Description of the Prior Art
In recent years, much active study has been made, in many quarters, of photosensitive compositions as a means for achieving matters of social concern, such as absence of environmental pollution, economy of natural resources and energy economy and the produced photosensitive compositions have been put to practical use. Some of them have been applied to solder-masking inks for use in printed circuit boards, electric insulating inks, acid resisting inks, etc., and they are commercially available as ultra-violet curable type inks. The material for the formation of an ultra-violet curable type protective film for use in printed circuit boards is required to possess a number of practical characteristics. For instance, the solder mask formed selectively on a tin-lead plated circuit board made of epoxy-fiberglass composite is required to be characterized such by features that: it can attain a thickness sufficient to fully cover the circuit by the aid of screen printing and, even in a thick-film state; it is curable upon exposure to a short time ultra-violet irradiation; it has a film hardness suitable for masking purposes; it possesses heat resistance, flexibility and adhesion sufficient that the film can follow the fluidity of solder melted again at the time of soldering, thereby maintaining adhesion and the appearance of the film after soldering; and it possesses solvent resistance at the time of washing the flux residue, oil and so forth, and it possesses electric insulation and moisture-resistance required for the protective film and the like.
However, the fact is that it is difficult to obtain a composition capable of exhibiting the electrical, mechanical, thermal and chemical properties essential to the permanent protective film for use in printed circuit boards owing to the general defects resulting from the unevenness of the curing and the stress of the film that has been cured rapidly mainly by a light energy-utilizing radical reaction and in view of the circumstances where photosensitive compositions have hitherto been developed with the intention of being utilized as image forming materials, typically, for relief plates and intaglio plates and the like, as well as ornamental materials for wood, paper, plastic and the like.
Although various photosensitive compositions are well-known, only a small number have taken account the mechanical, chemical, physical and electric properties to such an extent as to make them applicable as a permanent protective film for use in printed circuit boards, and to meet the conditions for safety, sanitation and curability. Epoxy acrylate is utilized for various kinds of ink vehicles because it possesses superior characteristics such as photosensitivity, hardness, solvent resistance and heat resistance. U.S. Pat. Nos. 3,772,062 and 4,014,771 disclose that improvements in curing speed, hardness, solvent resistance and so forth are achieved by the combination of a relatively low molecular epoxy acrylate with a monofunctional monomer or polyfunctional monomer, but not in the properties required for protective film materials for use in printed circuit boards, such as electric insulation, resistance, adhesion and so forth. Generally speaking, epoxy acrylate is defective in that it is inferior in respect of the properties such as flexibility and adhesion, and accordingly ingenuity should be exerted in overcoming this defect. It is well known that compounding of said monofunctional monomer in a large quantity is not preferable from the viewpoint of safety and sanitation, and it is apt to deteriorate the physical property of the coated film generally, with the exception of specific uses. U.S. Pat. No. 4,003,877 proposes a composition capable of functioning as protective film materials for use in printed circuit boards without utilizing a monofunctional monomer. However, said patent is silent as to improving the aforesaid defects inherent in the epoxy acrylate. It involves many difficulties to attain flexibility by the individual use of an epoxy acrylate-polyfunctional monomer system. U.S. Pat. No. 4,169,732 discloses the attainment of flexibility and coated film strength by the concurrent use of relatively high molecular weight bisphenol A type and novolak type epoxy acrylates, but said patent is different from the instant invention in view of employing the post-curing and organic solvent.
A urethane modified acrylate, generally speaking, is superior in the properties of flexibility and adhesion, but is inferior in heat resistance, chemical resistance, moisture resistance, hardness and ultra-violet curability as compared with said epoxy acrylate. Prior inventions are mostly directed toward improvements in this defect. For instance, U.S. Pat. No. 4,072,770 is concerned with improvements in the mechanical properties of polyester urethane acrylate. However, this advantage is not developed to such an extent as to make it applicable to printed circuit boards. Further, it is difficult to attain a sufficient usability as the permanent protective film for use in printed circuit boards by the individual use of said urethane modified acrylate. In U.S. Pat. Nos. 3,883,352 and 4,018,940 there are disclosed polyene-polythiol compositions which are relatively superior in coated film flexibility as well as superior in heat resistance and adhesion. However, polythiol type vehicles in general are defective in that they emit a strong foul odor.